The heart of all switched mode power supplies (“SMPS”) is the dc to de converter stage. While various switching and circuit topologies are available, a transformer effective to transfer power at the switching frequency is always provided for two reasons. The first is to make use of the transformer primary to secondary turns ratio to secure the required output voltage at maximum efficiency, and the second is to secure isolation between the input and output of the supply.
The converter transformer may have one or more primary windings which may be connected so as to vary the effective transformer turns ratio as a function of input frequency as disclosed in applicant's U.S. Pat. No. 5,907,236. The transformer secondary may have a single winding or be centre tapped
In many SMPS applications switching noise on the de output must be minimised. Regulations limiting “de conducted enissions” are being promulgated in Europe. Switching noise is conveyed to the de output through the converter transformer. One technique for minimising common mode noise transfer is to screen the primary and secondary windings. A third screen connected to chassis earth may be included between the two windings to minimise input-output capacitance. This is shown in FIG. 1. This ensures that noise signals are returned to the noise sources through the shortest possible path, rather than appear between the transformer secondary winding and chassis earth. This technique means the transformers are expensive. Apart from the additional screening materials the construction of shielded transformers is labour intensive and reliability may be compromised if the terminations between the grounding leads and the shields are not of high quality and have the potential to make electrical contact with any of the windings. Shields also result in reduced efficiency, due to eddy current losses.
An alternative technique for reducing common mode noise on the do supply would be to connect a capacitor of suitable value between the transformer secondary circuit and the transformer primary circuit. This would form a capacitance divider with the transformer primary to secondary capacitance and so reduce both the voltage and impedance of the noise source. However the provision of such a capacitor greatly increases the input-to-output capacitance of the supply and in some applications will exceed the maximum input-to-output capacitance allowable. Further, in many cases the input-to-output capacitance of the transformer alone may make desired minimums difficult to attain